Methods of fabricating planar PIN and APD photodiodes

ABSTRACT

In one aspect the invention relates to a high bandwidth shallow mesa semiconductor photodiode responsive to incident electromagnetic radiation. The photodiode includes an absorption narrow bandgap layer, a wide bandgap layer disposed substantially adjacent to the absorption layer, a first doped layer having a first conductivity type disposed substantially adjacent to the wide bandgap layer, and a passivation region disposed substantially adjacent to the wide bandgap layer and the first doped layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/245,902, filed on Nov. 3, 2000. Additionally, thisapplication is a divisional of, and claims priority to U.S. patentapplication Ser. No. 10/002,909, filed on Nov. 2, 2001, now U.S. Pat.No. 6,756,613, the entire disclosure of which is incorporated byreference herein.

FIELD OF THE INVENTION

This invention relates generally to semiconductor devices, and morespecifically to the structure of PIN photodiodes and APDs.

BACKGROUND OF THE INVENTION

For high-bit-rate, long-haul fiber-optic communicationsp-doped/intrinsic/n-doped (PIN) photodiodes and avalanche photodiodes(APDs) are frequently used as photodetectors due to their highsensitivity and bandwidth. Planar and mesa structures are two commonlyused configurations for PIN Photodiodes and APDs. Mesa structure PINphotodiodes and APDs are sometimes grown by molecular beam epitaxy (MBE)or metalorganic chemical vapor deposition (MOCVD). These fabricationtechniques allow the thickness of the layers and the wafer to beaccurately controlled.

Referring to FIG. 1, a mesa structure PIN 2 known in the prior art isshown. The structure includes a top metal contact 8, two bottoms metalcontacts 12, a p-doped Indium Gallium Arsenide (InGaAs) ohmic contactlayer 64 lattice matched to Indium Phosphide (InP), a p-doped InP layer68, an intrinsic narrow bandgap InGaAs absorption layer 76 latticematched to InP, a n-doped InP layer 80, and passivation regions 32.

In fabrication after the layers 80, 76, 68, 64 are sequentiallydeposited, the mesa structure 84 is formed by chemical etching throughthe p-doped layers 64, 68 and the intrinsic absorption layer 76. Next,the exposed sidewalls of the p-doped layers 64, 68 and the intrinsicabsorption layer 76 that define the mesa structure 84 are passivatedwith dielectric materials, such as SiO₂ or SiN_(x). As part of thisprocess, defects are inevitably introduced into the p-doped layers 64,68 and the intrinsic absorption layer 76. The intrinsic InGaAsabsorption layer 76 has a low bandgap and the mesa etching introduceddefects create extra intraband energy levels. These in turn lead to ahigh dark current. The dark current in InGaAs PIN photodiodes and APDsfabricated according to the above method is one factor in the generallylow reliability of these devices. The low reliability of these devicesincludes low sensitivity and high noise. These disadvantagessignificantly restrict the use of InGaAs PIN photodiodes and APDs inoptical communications systems.

Referring to FIG. 2, a planar structure PIN photodiode 4 known in theprior art is shown. The structure 4 includes a top metal contact 8, twobottom metal contacts 12, an intrinsic InGaAs layer 16, an intrinsic InPlayer 20, an intrinsic absorption InGaAs layer 76, a n-doped InP layer28, passivation regions 32, a p-doped InGaAs diffusion region 36, and ap-doped InP diffusion region 40.

During fabrication of the planar structure PIN photodiode 4, the n-dopedInP layer 28, the intrinsic InGaAs layer 76, the intrinsic layer InP 20,and the intrinsic InGaAs layer 16 are sequentially deposited. Thep-doped regions 36 and 40 are then formed by diffusing, for example,Zinc (Zn) or Cadmium into the top central region of the device 4. Afterthe diffusion step, the top metal contact 8 and the passivation regions32 are added.

Although avoiding the introduction of defects into the intrinsic InGaAslayer 76 during passivation, planar structure PIN photodiodes 4 havedisadvantages in device performance and design flexibility. Theintroduction of the p-dopant by diffusion is not a precise process, and,therefore, the thickness of the p-doped regions 36 and 40 cannot beaccurately controlled. In some instances the p-dopant diffuses into theintrinsic InGaAs layer 76. In other instances the p-dopant does notdiffuse completely through the intrinsic InP layer 20, or even throughthe intrinsic InGaAs layer 76. Another disadvantage of planar structurePIN photodiodes 4 is their higher parasitic capacitance. The parasiticcapacitance exists between the conductive substrate and device pad. Mesastructure devices can avoid this problem, however, by employing asemi-insulating substrate.

An additional disadvantage of planar structure PIN photodiodes 4 is thattheir fabrication process is complex. In particular, the diffusionprocess requires that the surface of the layer to be doped be carefullyprepared. A further disadvantage of planar structure PIN photodiodes 4is the control of hazardous materials as part of the dopant diffusion.For example, in Zn diffusion, As, P, Zn₃P₂, and Zn₃As₂, are heated toapproximately 550C. At this temperature, small evaporated and inhaleddoses are lethal.

What is needed are PIN photodiodes and APDs that overcome thedisadvantages of current PIN photodiodes and APDs.

SUMMARY OF THE INVENTION

In one aspect the invention relates to a high bandwidth shallow mesasemiconductor photodiode responsive to incident electromagneticradiation. The photodiode includes an absorption narrow bandgap layer, awide bandgap layer disposed substantially adjacent to the absorptionlayer, a first doped layer having a first conductivity type disposedsubstantially adjacent to the wide bandgap layer, and a passivationregion disposed substantially adjacent to the wide bandgap layer and thefirst doped layer.

In one embodiment, the photodiode also includes a second doped layerdisposed substantially adjacent to the absorption narrow bandgap layer.In another embodiment the photodiode also includes a third doped layerdisposed substantially adjacent to the first doped layer and adapted toform an ohmic contact with a substantially adjacent metalization layer.In an additional embodiment, the photodiode also includes a second dopedlayer and an impact layer disposed substantially adjacent to the seconddoped layer and the absorption narrow bandgap layer. The ratio of theionization coefficient for electrons relative to the ionizationcoefficient for holes for the impact layer is larger than thecorresponding ratio for the absorption narrow bandgap layer, the widebandgap layer, the first doped layer, and the second doped layer.

In a further embodiment, the first doped layer includes indiumphosphide. In yet another embodiment, the absorption layer comprisesindium gallium arsenide. In yet an additional embodiment, the widebandgap layer varies in thickness from a deposition thickness t₁ to anetching thickness t₂.

In another aspect the invention relates a method for fabricating highbandwidth shallow mesa semiconductor photodiode responsive to incidentelectromagnetic radiation. The method includes generating an absorptionnarrow bandgap layer, generating a wide bandgap layer disposedsubstantially adjacent to the absorption narrow bandgap layer,generating a first doped layer disposed substantially adjacent to thewide bandgap layer. The first doped layer has a first conductivity type.The method also includes etching a region of the first doped layer,etching a region of the intrinsic wide bandgap layer, and generating apassivation layer disposed substantially adjacent to the first dopedlayer and the intrinsic wide bandgap layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be betterunderstood by referring to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a mesa PIN photodiode as known to the prior art;

FIG. 2 is a planar PIN photodiode as known to the prior art;

FIG. 3 is an embodiment of a PIN photodiode according to the invention;

FIG. 4 is an embodiment of an avalanche photodiode according to theinvention;

FIG. 5 is a flowchart representation of a method for fabricating ashallow-mesa PIN photodiode according to the invention; and

FIG. 6 is a flowchart representation of a method for fabricating ashallow-mesa APD according to the invention.

DETAILED DESCRIPTION

Referring to FIG. 3, one embodiment of a shallow mesa planar PINphotodiode 60 according to the invention is shown. The PIN photodiode 60includes a top metal contact 8, two bottoms metal contacts 12, a p-dopedIn_(0.53)Ga_(0.47)As ohmic contact layer 64 latticed matched to InP, ap-doped InP layer 68, a wide bandgap intrinsic InP layer 72, anintrinsic narrow bandgap In_(0.53)Ga_(0.47)As absorption layer 76lattice matched to InP, a n-doped InP layer 80, and passivation regions32.

In an alternative embodiment, the wide bandgap intrinsic InP layer 72 isreplaced with Indium/Aluminum/Arsenide (In_(x)Al_(1−x)As) latticematched to InP. In one embodiment, the shallow mesa planar PINphotodiode 60 operates according to the principles of back illumination.In this embodiment, the photons in an incident beam pass through then-doped InP layer 80 and into the intrinsic In_(0.53)Ga_(0.47)Asabsorption layer 76. In various other embodiments, the thickness of thelayers and the dopant concentrations are selected according to Table 1.

TABLE 1 Layer Thickness Dopant Concentration p-In_(x)Ga_(1−x)As layer 6450 nm 1 × 10¹⁸–5 × 10¹⁸ cm⁻³ p-InP layer 68 0.2–0.4 um 1 × 10¹⁸–5 × 10¹⁸cm⁻³ i-InP layer 72 0.2–0.4 um Not Applicable i-In_(0.53)Ga_(0.47)Aslayer 76 1–3 um Not Applicab1e n-InP layer 80 0.5–1 um 1 × 10¹⁸–5 × 10¹⁸cm³

As part of fabrication the p-doped In_(0.53)Ga_(0.47)As layer 64, thep-doped InP layer 68, the intrinsic InP layer 72, the intrinsicIn_(0.53)Ga_(0.47)As layer 76, and the n-doped InP layer 80 aredeposited by MBE or MOCVD techniques. This means that the thickness ofthe layers 64, 68, 72, 76, 80 can be accurately controlled. Once all thelayers 64, 68, 72, 76, 80 have been deposited, the mesa 84′ is formed byetching through the p-doped In_(0.53)Ga_(0.47)As layer 64 and thep-doped InP layer 68, and into the intrinsic InP layer 72. The etchingprocess is controlled so that after completion the intrinsic InP layer72 has a thickness t₁ in the range of 0.1–0.3 um. The lower bound onthis range ensures that the intrinsic In_(0.53)Ga_(0.47)As layer 76 isadequately protected from the introduction of defects from thepassivation process. The deposition thickness t₂ of the intrinsic InPlayer 72 is chosen so as to minimize the carrier-transit time increaseintroduced by the additional layer 72.

In general the introduction of defects into the intrinsic InP layer 72during the passivation process does not lead to significant surfaceleakage current. This is due in part to the wideband gap of InP. Inaddition, potential dark current from this layer 72 is minimized byclosely monitoring the etching process so that little of the sidewall ofthe intrinsic InP layer 72 is exposed. The passivated region of the InPlayer 72 away from the mesa does not produce significant dark currentbecause the electric field is relatively weak in this region. Similarly,the dark current from the p-doped In_(0.53)Ga_(0.47)As layer 64 and thep-doped InP layer 68 is not significant because the electric field inthese regions is low.

Due to the confined lateral extent W of the mesa 84′, the electric field92 is confined below and within the mesa 84′. This design featuredefines the photosensitive region of the intrinsic In_(0.53)Ga_(0.47)Aslayer 76, that is, the area of the intrinsic In_(0.53)Ga_(0.47)As layer76 containing the electric field 92.

Referring to FIG. 4, one embodiment of a shallow mesa planar APD 120according to the invention is shown. The structure and fabrication ofthe upper portion of the APD 120 is similar to the shallow mesa planarPIN photodiode 60 discussed for FIG. 3. In particular, the upper portionincludes a top metal contact 8, post-etching passivation regions 32, ap-doped In_(0.53)Ga_(0.47)As layer 64, a p-doped InP layer 68, anintrinsic InP layer 72, and an intrinsic In_(0.53)Ga_(0.47)As absorptionlayer 76.

The lower portion of the APD photodiode 120 includes an intrinsicInAlGaAs layer 124, a p-doped InAlAs layer 128, an intrinsic InAlAslayer 132 latticed matched to InP, a n-doped InP layer 80, and twobottom metal contacts 12. The intrinsic InAlGaAs layer 124 is presentfor bandgap matching purposes. The p-doped InAlAs layer 128 is presentto assist in the modulation of the electric field. The intrinsic InAlAslayer 132 provides a region of large electric field to drive theelectron impact ionization avalanche process. This is achieved in theAPD 120 because the ratio of the ionization coefficient for electronsrelative to the ionization coefficient for holes for the intrinsicInAlAs layer 132 is large with respect to the ratios of the other layers64, 68, 72, 76, 124, 128, 80.

In alternative embodiments of the shallow mesa planar APD 120, thethickness of the layers 64, 68, 72, 76 is varied as described above inTable 1. In an alternative embodiment, the wide bandgap intrinsic InPlayer 72 is replaced with Indium/Aluminum/Arsenide (In_(x)Al_(1−x)As)lattice matched to InP. In one embodiment, the shallow mesa planar APD120 operates according to the principles of back illumination. In thisembodiment, the photons in an incident optical beam pass through then-doped InP layer 80 and into the intrinsic In_(0.53)Ga_(0.47)Asabsorption layer 76. In various embodiments, the thickness of the layers124, 128, 80 and their dopant concentrations are selected according toTable 2.

TABLE 2 Layer Thickness Dopant Concentration i-InAlGaAs layer 124 0.25um Not Applicable InAlAs layer 128 0.2–0.5 um 1 × 10¹⁸–5 × 10¹⁸ cm⁻³i-InAlAs layer 132 0.2–0.5 um Not Applicable n-InP layer 80 0.5–1 um 1 ×10¹⁸–5 × 10¹⁸ cm⁻³

Referring to FIG. 5 a flowchart representation of a method 145 forfabricating a shallow-mesa PIN photodiode according to the invention isshown. The method 145 includes generating an absorption narrow bandgaplayer (step 150), for example in one embodiment intrinsicIn_(0.53)Ga_(0.47)As, and generating a wide bandgap layer (step 155),for example in one embodiment intrinsic InP, substantially adjacent tothe narrow bandgap layer. The method 145 also includes generating afirst doped layer (step 160), for example in one embodiment p-doped InP,substantially adjacent to the wide bandgap layer. The first doped layerhas a first conductivity type. The thickness of the first doped layer isdetermined in part according to the etching accuracy and is generallysmall compared to the other layers in order to minimize the carriertransit time increase introduced by its presence. The method 145additionally includes etching a region of the first doped layer (step165) and etching a region of the intrinsic wide bandgap layer (step170). The method 145 further includes generating a passivation region(step 175) disposed substantially adjacent to the first doped layer andthe intrinsic wide bandgap layer. In etching the intrinsic wide bandgaplayer, the processes of step 170 are designed to ensure that an adequatethickness of the first doped layer remains to protect the absorptionnarrow bandgap layer from defects introduced during the passivation step175.

In one embodiment, the method 145 also includes generating a seconddoped layer (step 180) disposed substantially adjacent to the absorptionnarrow bandgap layer. In another embodiment, the method 145 alsoincludes generating a third doped layer (step 185) disposedsubstantially adjacent to the first doped layer and adapted to form anohmic contact with a substantially adjacent metalization layer.

Referring to FIG. 6 a flowchart representation of a method 190 forfabricating a shallow-mesa APD according to the invention is shown. Theoperation of the steps 150 through 175 is as described above withrespect to FIG. 5. The method shown in FIG. 6 also includes generating asecond doped layer (step 195) and generating an impact layer (step 200)disposed substantially adjacent to the second doped layer and theabsorption narrow bandgap layer. The impact layer is chosen so that theratio of the ionization coefficient for electrons relative to theionization coefficient for holes for the impact layer is larger than thecorresponding ratio for the absorption narrow bandgap layer, the widebandgap layer, the first doped layer, and the second doped layer. In oneembodiment, the method 190 also includes generating a third doped layer(step 205) disposed substantially adjacent to the first doped layer andadapted to form an ohmic contact with a substantially adjacentmetalization layer.

Those skilled in the art will recognize that the PIN and APD structuresin FIGS. 3 and 4, respectively, each represent only a single PIN and APDembodiment and that the principles of the invention can equally well beapplied to alternative PIN and APD structures known in the art.

Having described and shown the preferred embodiments of the invention,it will now become apparent to one of skill in the art that otherembodiments incorporating the concepts may be used and that manyvariations are possible which will still be within the scope and spiritof the claimed invention. These embodiments should not be limited todisclosed embodiments but rather should be limited only by the spiritand scope of the following claims.

1. A method for fabricating a shallow mesa semiconductor photodiode,comprising the steps of: generating an absorption narrow bandgap layer;generating a wide bandgap layer disposed substantially adjacent to theabsorption narrow bandgap layer; generating a first doped layer disposedsubstantially adjacent to the wide bandgap layer, the first doped layerhaving a first conductivity type; etching a region of the first dopedlayer; etching a region of the wide bandgap layer; and generating apassivation region disposed substantially adjacent to the first dopedlayer and the intrinsic wide bandgap layer.
 2. The method of claim 1further comprising generating a second doped layer disposedsubstantially adjacent to the absorption narrow bandgap layer.
 3. Themethod of claim 2 further comprising generating a third doped layerdisposed substantially adjacent to the first doped layer and capable offorming an ohmic contact with a substantially adjacent metalizationlayer.
 4. The method of claim 1 further comprising: generating a seconddoped layer; and generating an impact layer disposed substantiallyadjacent to the second doped layer and the absorption narrow bandgaplayer, wherein the ratio of the ionization coefficient for electronsrelative to the ionization coefficient for holes for the impact layer islarger than the corresponding ratio for the absorption narrow bandgaplayer, the wide bandgap layer, the first doped layer, and the seconddoped layer.
 5. The method of claim 4 further comprising generating athird doped layer disposed substantially adjacent to the first dopedlayer and capable of forming an ohmic contact with a substantiallyadjacent metalization layer.
 6. The method of claim 1 wherein the firstdoped layer comprises indium phosphide.
 7. The method of claim 1 whereinthe absorption layer comprises indium gallium arsenide.
 8. The method ofclaim 1 wherein the wide bandgap layer varies in thickness from anetching thickness t₁ to a deposition thickness t₂.
 9. The method ofclaim 1 wherein the photodiode is adapted to receive incident light in adirection perpendicular to two or more of the plurality of layers thatcomprise the photodiode.